Immunotherapy represents a recent breakthrough in cancer treatment fueled by the accelerating mechanistic understanding of how transformed cells subvert our immunosurveillance. Therefore, developing therapeutic strategies that enhance systemic immunosurveillance in a controllable manner has become one of the major interests in this field. Among different agents, small molecule activators (agonists) of the cGAS-STING pathway have recently attracted attention because they are expected to synergize with immunotherapies and enhance anti-cancer immune response by upregulating the interferon response. We previously identified a compound, BDW568, that was shown to activate the interferon pathway in a STING-dependent manner. Surprisingly, follow-up studies demonstrated that cGAS and STING are not direct target(s) of BDW568. Therefore, observed phenotype suggests that BDW568 acts either through (1) binding to an unknown regulator of the cGAS-STING pathway, or (2) by generating an unknown signaling molecule independent of 2?,3?-cGAMP, the endogenous STING agonist. In the proposed study we will identify the cellular target of BDW568 responsible for the previously discovered phenotype using two orthogonal strategies. In the first approach, we will synthesize BDW568-based photoaffinity probes and use them to label any protein that binds to BDW568. Labeled target(s) will be identified by a whole-cell lysate pulldown experiments coupled with mass spectrometry (MS). In the second approach, we will use CRISPR-Cas9 to individually knock out all the genes that are known to interact with STING or associate with interferon pathways. The knockout of the BDW568 target should demonstrate either resistance to the compound or elevation of the basal interferon level without BDW568 through STING. Any target candidate that emerges from either one of these approaches will be rigorously validated according to the standards of the field to establish that observed phenotype is due to on-target engagement. Successful completion of proposed studies will expand the target space within cGAS-STING pathway. These insights will lead to additional opportunities for developing adjuvant immunotherapies as well as expand our understanding of innate immune response in mammalian cells.